The invention concerns a device for supporting a horizontal guided, continuous glass strand.
The Danner-Technique and the Vello-Technique are used for the industrial production of glass rods and glass tubes up to diameters of approximately 100 mm. With the Danner-Technique the glass flows from a nozzle on top of a rotating tilted pipe, through which, in the case of producing tubes, a glass is blown and a glass strand is pulled off continuously from the tip. With the Vello-Technique a ring nozzle, through whose center the glass is blown for the tubing production, creates the glass strand.
Tube drawing techniques have become known for example from DE 100 54 804 A1.
With the usual techniques for the production of glass rods or glass tubes, the hot and ductile glass strand is directed into the horizontal level, cooled down over the length of the drawing bench and finally separated into sections of the desired length. The glass strand, which is still hot, is supported along the drawing path. This takes place usually by means of rollers or supports from a material, which leaves fewest possible traces on the surface of the glass strand.
Wood and graphite are the materials usually used for these rollers. V-shaped blocks made from the same materials are used instead of the rollers if extremely quiet running of the glass strand is demanded within the drawing path; the recess of the V-shaped blocks guides the glass strand.
Good lateral guide properties are achieved while accepting that the strand slides on the support and that there is the danger of developing scratches.
The small supporting surface, which the rollers offer to the glass strand, causes the glass strand (especially in its hot area) to be deformed during transport over the rollers, which in return causes an impairment of its dimensional accuracy and especially problems with ovalness. Furthermore, heat is removed from the glass strand along the contact line to the rollers. The nonuniform heat distribution over the circumference of the glass strand leads with further cooling to distortion and bending of the glass strand. In addition, the contact to the rollers transfers dust and dirt on the glass strand.
The guide rollers and guide blocks are especially in the hot area subjected to distinct wear by abrasion. This abrasion creates contaminations on the glass strand surface. During the progressive wear the occurrence of the scratches created by the supporting device increases, until the rollers and blocks must be exchanged after some time.
DE 31 25 521 A1 describes a device for conveying and supporting a hot, continuous glass tube. The supporting device exhibits a V-shaped recess, which forms a supporting area for the glass tube. The supporting area exhibits drilled holes in the vertex of the V, which are connected to compressed air. Compressed air is to lift the glass tube off the supporting area. However, in the range of the legs of the V-shaped supporting area approximation of the glass tube to the walls of the legs takes place and therefore also contact.
This leads to a mutual impairment. On the one hand scratches can develop on the exterior of the glass tube, on the other hand material wear of the supporting device takes place, especially if a soft material such as graphite is used for it.
A prior art design variation is known in which the glass tube strand is guided between two walls bent against each other, which are usually made of sheet metal, and is carried on air, which discharges from the gap between the walls. Usually these devices are present only in the first few meters; rollers do the further transport of the glass strand. In such devices one tries to stabilize the glass strand centered on a gas cushion by means of suitably guiding the gas that is emerging from a gap under the glass strand. However, lateral contact of the glass strand with the walls cannot always be prevented, so that in these cases the surface of the glass strand is damaged or contaminated.
A freely floating guide of the freshly formed glass strand on compressed air nozzles would be possible. In this case no contact can occur between glass strand and firm materials. However, the necessary airflow for supporting the glass strand would be so large, that the nozzles would cause an inadmissibly high noise level. Furthermore there is the risk that the gases impacting the glass strand surface with high speeds deform the glass strand especially in its hot area. Besides, this principle is not economical due to the high costs of compressed air.